This invention pertains to a monitor for the overspeed protection system in a gas turbine engine.
In the prior art gas turbine engines, a mechanical overspeed governor was used. The purpose of the governor is to bypass fuel if engine speed exceeds some particular value. Usually this value has been 104% of rated engine rpm. In the mechanical overspeed governor, flyweights were used to operate a pilot valve controlling the servo fluid pressure to a power piston. This piston controls a bypass valve which shunts fuel away from the engine beginning at about 100.4 percent engine speed. As engine speed increases, the bypass valve opens further and, at 106 percent engine speed, fuel is completely bypassed from the engine.
Later, electronic engine overspeed controls replaced the mechanical overspeed governors and made it possible to increase reliability through the use of redundant speed sensors and associated circuitry. These sensors count teeth on gears placed in the power turbine bearing package. A fuel shut-off valve is actuated by the overspeed controller when an overspeed condition is sensed. To insure the increased reliability of the redundant speed pickups, it is necessary to conduct periodic checks of the multiple speed sensors at intervals of 100 to 200 flights. With the prior art electronic overspeed system, these tests are made in the following way. The aircraft is moved to a maintenance engine run-up area and a hand held test set is connected to each engine. The engine must then be started and run at a speed sufficient to actuate the speed sensing system, that is a fan speed between 35% and 50% of rated speed. This procedure is expensive in manpower, requires personnel to be exposed to engines operating above the ground idle condition, causes additional starts and decreases aircraft availability. These costs are largely eliminated by the addition of the overspeed monitor which we have invented. Our overspeed monitor is small, lightweight and is installed on the engine at all times. It monitors the function of the speed pickups throughout every flight and when inspected visually, with the engine shutdown, indicates the performance of the overspeed control system throughout the previous flight. With our invention, the ground run-up inspection procedure is eliminated. A static hangared inspection requires less time than an oil level check.